People who are near-sighted can only see things that are very close to their eyes.' Everything else is not so clear. Many people who do a lot of close work, such as writing, reading and sewing, become near sighted. Then they have to wear glasses in order to see distant (远处的) things clearly.
People who are far-sighted suffer from just the opposite problem. They can see things that are far away, but they have difficulty in reading a book unless they hold it at arm's length. If they want to do much reading, they must get glasses, too.
Other people do not see clearly because their eyes are not exactly the right shape. They have what is called astigmatism (散光). This, too, can be corrected by glasses. Some people's eyes become cloudy because of cataracts (白内障). Long ago these people often became blind. Now, however, it is possible to operate on the cataracts and remove them."
Having two good eyes is important for judging distances. Each eye sees things from a slightly different angle (角度). To prove this to yourself, look at an object out of one eye; then look at the same object out of the other eye. You will find the object's relation to the background (背景) and other things around it has changed. The difference between these two different eye views helps us to judge how far away an object is. People who have only one eye cannot judge distance as people with two eyes.
We should take good care of our eyes ______.

A. only when we cannot see perfectly
B. only when we can see well
C. even if we can see well
D. only when we realize how important our eyes are